A chemically modified polypeptide in which at least one of amino groups, carboxyl groups, mercapto groups or guanidino groups in the polypeptide molecule is modified with a polyalkylene glycol derivative (WO 95/023165) and modification of free thiol groups of cysteine residues in the polypeptide molecule are known (EP 0668353). However, when at least one of such amino groups, carboxyl groups, mercapto groups, guanidino groups or free thiol groups of cysteine residues in the polypeptide molecule is modified with a polyalkylene glycol derivative, the activity of the polypeptide may be markedly or completely lost.
For example, the activity of interleukin-15 completely disappears when its amino group(s) are modified with polyethylene glycol [J. Biol. Chem., 272:2312 (1997)].
Nothing is known about a chemically modified polypeptide in which at least one of the hydroxyl groups in the polypeptide molecule is modified with a polyalkylene glycol derivative.
Great attention has been directed toward the development of    (1) a method for the analysis of influences of hydroxyl groups upon the activity of a polypeptide, in a case where the hydroxyl group concerned is in the active site of the polypeptide,    (2) a novel chemical modification method which can avoid a probable case where the biological activity of a polypeptide is considerably spoiled when the polypeptide is treated by a conventional chemical modification method, and a chemically modified polypeptide obtained by the method, and    (3) a novel method which can improve resistance of polypeptide against protease, freezing-thawing and denaturing agents.